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There are multiple pKVM memory transitions where the state of a page is not cross-checked from the completer's PoV for performance reasons. For example, if a page is PKVM_PAGE_OWNED from the initiator's PoV, we should be guaranteed by construction that it is PKVM_NOPAGE for everybody else, hence allowing us to save a page-table lookup. When it was introduced, hyp_ack_unshare() followed that logic and bailed out without checking the PKVM_PAGE_SHARED_BORROWED state in the hypervisor's stage-1. This was correct as we could safely assume that all host-initiated shares were directed at the hypervisor at the time. But with the introduction of other types of shares (e.g. for FF-A or non-protected guests), it is now very much required to cross check this state to prevent the host from running __pkvm_host_unshare_hyp() on a page shared with TZ or a non-protected guest. Thankfully, if an attacker were to try this, the hyp_unmap() call from hyp_complete_unshare() would fail, hence causing to WARN() from __do_unshare() with the host lock held, which is fatal. But this is fragile at best, and can hardly be considered a security measure. Let's just do the right thing and always check the state from hyp_ack_unshare(). Signed-off-by: Quentin Perret <qperret@google.com> Acked-by: Will Deacon <will@kernel.org> Link: https://lore.kernel.org/r/20241128154406.602875-1-qperret@google.com Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.